Reduced engine operation times in hybrid vehicles, such as plug-in hybrid vehicles, enable fuel economy and reduced fuel emissions benefits. However, the shorter engine operation times can lead to insufficient purging of fuel vapors from the vehicle's emission control system. To address this issue, hybrid vehicles may include a fuel tank isolation valve (FTIV) between a fuel tank and a hydrocarbon canister of the emission system to limit the amount of fuel vapors absorbed in the canister. Engine control systems may coordinate fuel tank pressure relief with refueling and canister purging operations to enable emissions control.
One example approach of emissions control is shown by Kidokoro et al. in U.S. Pat. No. 6,796,295. Therein, during engine operation, the FTIV is opened if a fuel tank pressure exceeds a limit and if the canister purge rate is higher than a threshold, to return the tank pressure near atmospheric pressure values.
However, the inventors herein have identified a potential issue with such an approach. As one example, air-to-fuel ratio disturbances may arise since canister loading may be more variable (and less predictable) than canister unloading. The disturbances may be exacerbated during lower canister purge rate conditions. Specifically, since the FTIV is kept open until the desired fuel tank pressure is reached, the amount of fuel vapors bled from the fuel tank to the canister may vary unpredictably. For example, there may be sudden fuel vapor spikes during the unloading of fuel vapors from the canister. In one example, the fuel vapor spikes from the fuel tank may overload the canister leading to higher air-to-fuel ratio disturbances and degraded exhaust emissions.
Thus in one example, the above issue may be at least partly addressed by a method of operating a fuel vapor recovery system. In one example embodiment, the method comprises, purging fuel vapors from a canister to an engine intake to reduce a stored fuel vapor amount in the canister, and intermittently purging fuel vapors from a fuel tank to the canister to increase a stored fuel vapor amount in a canister buffer. Further, a duration and interval of the intermittent purging may be based on the stored fuel vapor amount in the buffer.
By adjusting the purging from the fuel tank based on a buffer capacity, loading of fuel vapors from the fuel tank to the buffer may be better controlled. In particular, by delivering fuel vapors as multiple purge pulses, rather than as a single purge, with each pulse adjusted based on the buffer capacity, buffer loading may be better controlled and air-to-fuel ratio disturbances may be reduced. By cyclically unloading a canister buffer before loading the buffer with fuel vapors from the fuel tank, purging of fuel vapors from the fuel tank may be better coordinated with purging of fuel vapors from the canister.
In one example, an engine may include a fuel vapor recovery system with a fuel tank isolation valve coupled between a fuel tank and a canister, and a canister purge valve coupled between the canister and the engine intake. During purging conditions, the canister purge valve may be opened, while the isolation valve is maintained closed, to purge fuel vapors from the canister to the engine intake until the amount of fuel vapors in the canister is below a threshold (e.g., until the canister is empty). As such, the canister may have a buffer region that is purged towards the end of the canister purging operation such that when the amount of fuel vapors in the canister is below the threshold, an amount of fuel vapors in the buffer is also reduced and a capacity of the buffer is increased above a threshold capacity.
When the amount of fuel vapors in the canister is below the threshold (e.g., empty), and the buffer capacity has increased, the fuel tank isolation valve may be intermittently opened (or pulsed) to purge fuel vapors from the fuel tank to the canister, specifically, to the buffer region of the canister. The total amount of fuel vapors that are purged from the fuel tank to the buffer may be based on the buffer capacity to allow the buffer to be refilled with fuel vapors, but not overfilled. The duration of each pulse, as well as an interval between consecutive pulses may be adjusted based on the amount of fuel vapors stored in the buffer (or the buffer capacity) at the onset of the intermittent purging from the fuel tank. The duration of pulses and/or interval between pulses may also be adjusted based on a fuel tank pressure at the onset of the intermittent opening, as well as canister purge rate.
In this way, overloading of the buffer is reduced, and overflow of fuel vapors from the buffer into the canister is reduced. By further adjusting the pulses based on the fuel tank pressure, fuel tank pressure may be maintained within limits without causing air-to-fuel ratio disturbances. As such, this leads to improved exhaust emissions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.